Currently, there were many reports about methods for preparing hydrorefining catalysts with Ni—Mo, Co—Mo, Ni—W, Co—W, Co—Mo—Ni, and W—Mo—Ni as active components. In the process of preparing the catalysts containing cobalt and molybdenum by impregnation method, a main problem that exists is that the active components Co and Mo are difficult to be dissolved, and especially two active components are difficult to be dissolved in one solvent. Further, in preparation of the catalysts by using a multi-step impregnation method, the preparation cost of the catalysts is increased. In preparation of the catalysts by using a one-step impregnation method, problems that exist in the prior art are as follows: an organic acid/base needs to be added to promote dissolution of the active components; when the contents of the active components are required to be high, a large amount of organic acid/base needs to be used; during calcination, the temperature is difficult to be controlled, and temperature runaway easily occurs, a large amount of gas is discharged, and loss of catalyst is serious.
Further, whether the formulated impregnation solution of active components is clear and homogeneous directly influences the dispersion degree of the active components in the catalyst after impregnation, thereby influencing the catalyst activity and selectivity. Therefore, selection of effective solvent and dissolution method is a key factor for formulation of the impregnation solution.
U.S. Pat. No. 4,409,131 discloses a method for preparing a CoMo/NiMo catalyst by impregnating a support in a solution containing active components and aqueous ammonia in one step. A process for formulating the impregnation solution is introduced in detail, in which it is required to heat a mixture to promote dissolution of the active components.
U.S. Pat. No. 6,013,598 discloses a method for preparing the catalysts for selective hydrodesulfurization. The catalyst includes cobalt, molybdenum as active components and alumina as a support, and is prepared by isometric impregnating the support in an aqueous solution of the active components with citric acid added, and the maximum content of molybdenum oxide in the resulting catalyst is merely 10 wt %.
CN91110935.8 discloses a method for preparing a hydrorefining catalyst, which includes dissolving cobalt acetate in water, adding ethylenediamine to form a mixed solution of cobalt ethylenediamine, adding ammonium molybdate to the solution at pH 12 to 14, to prepare a co-impregnation solution containing cobalt and molybdenum metals, and impregnating a porous support with the co-impregnation solution, so as to prepare the catalyst. This method has the disadvantage that calcination needs to be carried out in an oxygen-free or trace oxygen atmosphere, in order to obtain the catalyst product.
CN94114194.2 discloses a method for preparing a catalyst with zinc oxide-modified γ-alumina as a support and with cobalt and molybdenum as active components. The method includes: to a certain amount of aqueous ammonia with a concentration of 15% to 28%, adding a desired amount of a cobalt salt, and adding a certain amount of ethylenediamine under continuous stirring, till the amount of ethylenediamine was 1/50 to 1/7 based on the weight of the cobalt salt; and then adding a desired amount of ammonium molybdate under continuous stirring, to prepare a Co—Mo co-impregnation solution; impregnating the preparative support in the co-impregnation solution in one step for 1 to 10 hours, drying for 2 to 6 hours at 110° C. to 150° C., calcinating for 3 to 8 hours at 480° C. to 600° C., to prepare the catalyst, which preferably includes 1 wt % to 5 wt % CoO, 8 wt % to 14 wt % MoO, 1 wt % to 15 wt % ZnO, and the remaining being γ-Al2O3.
CN00130284.1 discloses a hydrorefining catalyst and method for preparing the same. The catalyst is prepared by impregnating an alumina support with a polymer and a Group IVB metal added, with a co-impregnation solution of ammonia containing molybdenum, cobalt, and nickel as active components. No detailed description of the process for formulating the impregnation solution is provided.
Currently, it is a well-known technology to use aqueous ammonia as a solvent to dissolve Co and Mo active components; however, the effect of aqueous ammonia to increase the dispersion degree of active components is not significant. It is still a challenge how to formulate a clear and stable impregnation solution containing the active components with a low surface tension and low viscosity in a simple way, where the requirement of one-step isometric impregnation of multiple components needs to be met, and after impregnation, the active components needs to keep a high dispersion state. For direct dissolution of a cobalt salt in water or aqueous ammonia without heating, if the amount of solvent is controlled, the cobalt salt tends not to be completely dissolved, and an additional organic acid or base needs to be added to accelerate dissolution of cobalt.
Because pyrolysis gasoline after two-stage hydrogenation is generally used as the raw material for aromatics extraction, the catalyst is required to have an excellent hydrogenation activity (bromine number of product of lower than 1.0 g Br2/100 g) and desulfurization activity (sulfur in product of lower than 1.0 μg/g), is also required to have a favorable selectivity for hydrogenation (avoiding loss in aromatic hydrogenation) and a favorable long-term operational performance. The catalysts for pyrolysis gasoline two-stage hydrogenation in current industrial units have a good hydrogenation activity and desulfurization activity, but suffered from insufficiency long-term operation capability, high loss rate in aromatic hydrogenation (above 2%), low high-load operation capability, and high production cost of catalyst, which influence the economic benefit of the unit.
The above problems are caused by the following main reasons:
(1) Due to poor dispersion of the active metal components on the catalyst, over-potent active center sites exist, resulting in high loss rate in aromatic hydrogenation, especially in benzene hydrogenation;
(2) Since a certain amount of B-acid centers exist on the catalyst, while the amount of strong L-acid centers of L-acids is high, the catalyst is easily coked at a high temperature, so that bed pressure drop is increased, circulation of hydrogen is influenced, hydrogen compressor cannot work, and unqualified hydrogenated products is produced, resulting in shutdown and reproduction;
(3) The specific surface and pore volume, especially the pore size distribution of the catalyst has great influence on the catalyst activity, so a suitable specific surface area, pore volume and pore size centralized in distribution are very important for mass transfer, heat transfer and diffusion;
(4) In order to improve the catalyst activity, the content of the active components may be increased, but a problem of poor dispersion occurs. In view of this problem, multi-step impregnation may be used in the preparation process of the catalyst, that is, after one-step impregnation, drying and calcination are performed, and then next impregnation is performed. As a result, the preparation process is complex, and waste of energy is serious, resulting in increased production cost of the catalyst.
Although one-step impregnation in which a complexing agent such as an organic acid, ammonia, or ethylenediamine is added in the formulation of the impregnation solution has been reported, addition of a large amount of macromolecular organic acid or base may make the subsequent calcination more difficult; and moreover, due to high volatility of ammonia, addition of ammonia may make the formulation of the impregnation solution more difficult, while the dispersion degree of the active components is not significantly increased.